Riblets are typically small surface protrusions, optionally aligned with a direction of fluid flow, which confer an anisotropic roughness to a surface. In the present application, the term “riblet” is broadly applied and does not connote any particular pattern or structure. That is, the riblets of the present invention may have any pattern or texture that confers roughness to a surface and facilitate the flow of fluid along that surface.
Riblet-enhanced surfaces can be used in a wide variety of applications such as airplanes, trains, missiles, and blades since riblet-enhanced surfaces can reduce the turbulent skin friction, particularly when aligned in a stream-wise direction.
In recent years, several manufacturing techniques have been applied to create the riblet structure such as grooving, molding, grinding, laser-beam-removing, lacquering and rolling.
U.S. Pat. No. 9,272,791 discloses a pipe for an aircraft refueling system. The pipe has an inner surface including a riblet texture. In this case the surface texture was formed from a layer of conductive material in the form of foil. The riblet structure on the surface of foil was produced by embossing. Then the inner surface was generated by wrapping a tape around the riblet foil. The riblet foil has a series of triangular grooves in which the height of riblets may be between 0.5 mm and 1.5 mm, and the spacing between riblets is between 0.75 mm and 2.25 mm. The riblet pattern can give rise to a drag reduction up to 12% due to a reduction in cross-flow turbulence intensity.
U.S. Pat. No. 9,151,415 discloses a pipe for transporting a multiphase oil/water mixture. The tubular pipe has an interior wall comprising two different regions having different affinities for oil and water. The interior wall may also comprise riblet structures oriented in the axial direction of the pipe. The interior wall is applied using several application mechanisms, including, for example, chemical vapor deposition, electroplating, thermal spray, liquid coatings, fusion bonded epoxy coatings, application tapes, roll-on tapes, photolithography, large scale self-assembly, and micro-patterning.
U.S. Pat. No. 5,386,955 discloses a variety of stream-wise riblet projections for drag reduction. In one case, a pattern in the surface consists of various larger-height projections between which are positioned a plurality of smaller-height projections. In another case the projections are arranged in a repeated span-wise pattern in which the adjacent projections in each pattern repeat to different heights.
A series of U.S. Pat. Nos. 7,070,850, 5,848,769, 5,133,516, 5,069,403, 4,986,496 disclose the drag reduction articles. These articles may be either single layer or multi-layers made from various materials in different shapes and sizes. As an application, these drag reduction articles may be adhesively bonded to the surface of an airplane so as to reduce the drag caused by a fluid flowing over the surface.
One important application for riblets is for aircraft wings or rotor blades for aerodynamic drag reduction. U.S. Pat. Nos. 8,413,928, 8,684,310, 8,668,166, and 8,678,316 describe these applications. Metal foils or other materials were processed to obtain riblet textures. U.S. Pat. Nos. 8,444,092 and 9,297,394 discloses metal sheets and plates with riblet textures manufactured by rolling processes. The metal can be aluminum alloy, titanium, or steel. These sheets and plates can be used in fabricating a portion of an aircraft or rotor blade.
U.S. Pat. No. 6,666,646 discloses a method called High Velocity Oxyfuel Process (HVOF) to make a riblet structure on a substrate from a powder. The riblet structure is applied on the gas turbine engine components for aerodynamic drag reduction.
U.S. Pat. No. 8,220,754 discloses a method to create or enhance a riblet structure by using a plasma. The riblet surface may be used to form part of an aircraft.
U.S. Pat. No. 4,907,765 discloses a wall with riblet textures for drag reduction. A photo-lithographic method or laser beam method is used for making such a wall.
U.S. Pat. No. 4,943,476 discloses an application of riblets to the fabrics used on machines in the papermaking industry. In the weaving of paper-making fabrics, fibers were used to form batts which are needled into the structure of fabric bearing a plurality of longitudinal grooves which can enhance their water-removing abilities.
Although riblet-enhanced surfaces may be fabricated using the above techniques, such methods are typically complex or high-cost which may limit their application to high value-added manufactured goods such as airplane wings or turbine blades. Accordingly, there is a need to develop a simplified method to obtain riblet structures, particularly riblet structures on large, difficult-to-reach surfaces such as the interior surface of a pipe.